Kuang Sheng

High Temperature Stability and the Performance Degradation of SiC MOSFETs

SiC MOSFET devices have great potentials in future high temperature power electronics applications due to their possible higher thermal runaway temperature compared with other SiC power semiconductor devices. In this paper, the high temperature stability of SiC MOSFETs is investigated by experiments and Saber simulations. The maximum steady-state junction temperature of the SiC MOSFET is measured to exceed 250 °C and saber simulations based on experimental model estimate that the thermal runaway temperatures are close to 300 °C. In addition, performance degradation of SiC MOSFETs during high-temperature operation is observed and discussed. Experimental results show that the degradation happens during both the high temperature storage (maximum 5% RON increment) and high temperature operation process (maximum 15% RON increment). The degradations are found to recover to a close-to-initial level after 1 h recovery time at the room temperature.

A Bidirectional LLC Resonant Converter With Automatic Forward and Backward Mode Transition

This paper proposes an improved bidirectional LLC resonant topology with a new control scheme. All the switches in the proposed topology can achieve soft switching. Compared with traditional isolated bidirectional dc-dc converters such as dual active bridge converter, the reverse energy and turn-off loss are reduced dramatically, and the conversion efficiency can be much improved. With the proposed new control scheme, the power flow direction and output power of the proposed converter can be changed automatically and continuously, which is attractive for energy storage systems to balance the energy and to keep the dc-bus voltage constant. Performance of the proposed circuit is validated by the experimental results from a 1-kW prototype. Over 97% efficiency is achieved at full load condition based on the prototype.

An All-SiC High-Frequency Boost DC–DC Converter Operating at 320 °C Junction Temperature

This letter presents the design, prototype development, operation, and testing of an 800 kHz, 1 kW, 800 V output boost dc-dc converter module that integrates SiC MOSFET and SiC Schottky diode die. It is observed that when the device loss is dominated by switching loss, the steady-state junction temperature of SiC MOSFET can reach as high as 320°C. This is the highest self-heated junction temperature operation of SiC power devices under room temperature ambient reported in the literature. The high-frequency switching characteristics and high-temperature thermal reliability of the assessed converter are evaluated in detail. A solder-molten phenomenon during high junction temperature operation is detected and the die-attachment material is thus improved to enhance the high-temperature thermal reliability of the converter module. This study shows that the high-frequency capability of a gate driver and high-temperature die-attachment technology can be limiting factors preventing SiC power devices from operating at higher junction temperatures.

A 3600 V/80 A Series--Parallel-Connected Silicon Carbide MOSFETs Module With a Single External Gate Driver

In this paper, a new series connection topology is introduced for silicon carbide (SiC) MOSFETs. In the topology, with a single external gate drive, three series-connected SiC MOSFETs are synchronously driven. The operating principle of the proposed topology is analyzed and presented. In order to improve the current capability of the module, parallel connection of two SiC devices are also demonstrated. A 3600 V/80 A series-parallel-connected configuration with three rows in a series and two branches in parallel is constructed with six 1200 V/40 A discrete SiC MOSFETs. Switching behavior of the configuration is completed at 2300 V/78 A. Experimental results verify the validity and feasibility of the proposed topology. Analysis based on experimental results for the circuit switching speed and switching losses is given. Finally, such a series-parallel-connected circuit is integrated in a SiC MOSFETs module, capable of 3600 V/80 A. The switching characteristics of the module are compared to the discrete configuration.

A Bidirectional Three-Level LLC Resonant Converter With PWAM Control

This paper proposes a bidirectional three-level LLC resonant converter with a new pulse width and amplitude modulation control method. With different control signals, it has three different operation modes with different voltage gains. Therefore, it can achieve wide voltage gain range by switching among these three modes, which is attractive for energy storage system applications needing wide voltage variation. The proposed topology operates with constant switching frequency, which is easy to implement with digital control, and it can achieve soft switching for all the switches and diodes in the circuit as a conventional LLC resonant converter. The performance of the proposed converter is validated by the experimental results from a 1-kW prototype with 20 A maximum output current.

Cryogenic and high temperature performance of 4H-SiC power MOSFETs

The electrical performance of 4H-SiC Power MOSFETs is studied at temperatures from 93K to 473K. With the decrease of operation temperature, the threshold voltage is found to increase linearly whereas the on-resistance shows a minimum value within the whole temperature range. The rapid increase of on-resistance at lower temperature is ascribed to the presence of large densities of interface traps at the SiC/SiO2 interface. In addition, the breakdown voltage is also measured down to 93K and found to increase monotonously with temperature. A set of parameters, determining the breakdown voltage of 4H-SiC MOSFET at different temperatures, is put forward in this paper, by fitting the experimental data.

A 10kV/200A SiC MOSFET module with series-parallel hybrid connection of 1200V/50A dies

In this paper, an innovative series-parallel hybrid circuit topology with silicon carbide MOSFETs is presented and analyzed. This topology consists of two uniform parts in parallel and each of the parts includes three sub-parts in series. The sub-part contains three primary parts connected in string where each of these primary parts has a parallel of two SiC MOSFETs. These 36 SiC devices are divided into six sub-parts, which are driven by a common driving signal. A 10kV/200A SiC MOSFETs module is fabricated based on this hybrid topology with thirty-six 1200V/50A SiC MOSFET dies. The dynamic switching behavior of the module is analyzed and double-pulse tests have been performed at 5400V/200A. The results show a good switching speed of 100ns in turn-on process and 200ns in turn-off process.

Fabrication of vertically stacked single-crystalline Si nanowires using self-limiting oxidation

A simple method for fabricating vertically stacked single-crystal silicon nanowires on standard bulk silicon wafers is presented. The process uses inductively coupled plasma (ICP) etching to create silicon fins with uneven yet controllable vertical profiles. The fins are then thermally oxidized in a self-limiting process, and the narrow regions are completely consumed to create multiple nanowires vertically stacked on each other. It was found that the number of nanowires in the vertical stack depends on the number of ICP cycles. A mechanism for the formation of the nanowires is proposed and confirmed with numerical simulations.

Enhancement-mode GaN-on-Silicon MOS-HEMT using pure wet etch technique

This paper reports for the first time a gate-recessed GaN-on-Silicon MOS-HEMT device with true normally-off operation and high breakdown voltage using a one step simultaneous oxidation/dissolving treatment by hybrid alkaline solution with hydrogen peroxide and potassium hydroxide. After 40-min wet etching at 95 oC solution temperature, the Al2O3/GaN MOS-HEMT device features a true normally-off operation with a threshold voltage of 3 V, extracted by the linear extrapolation of the transfer curve. Combined with the three-terminal off-state breakdown voltage up to 1492 V for the device with 28 μm gate-to-drain distance, this technique manifests an easy, stable and low cost approach for the commercialization of normally-off GaN power devices.

Design and experimental demonstration of 1.35 kV SiC super junction Schottky diode

This paper presents the first functional SiC super junction devices, SiC SJ Schottky diodes, which substantially improve the trade-off between the breakdown voltage and specific on-resistance in SiC power devices. Processes for fabricating SJ structures by using a trench-etching-and-sidewall-implant method has been developed and a functional SJ Schottky diode has been demonstrated based on this processing method. The measured cell blocking voltage was 1350V, which achieves 95% of the simulated blocking voltage for the ideally-charge-balanced SJ structure. The measured device specific on-resistance was 0.92mΩ·cm2. The SJ drift region specific on-resistance as low as 0.32 mΩ·cm2 was obtained after subtracting the substrate resistance.